1. Field of the Invention
The present invention relates to a switch apparatus that generates safety input signals to be inputted to a numerical control apparatus in order to monitor the safety function of a machine tool, and a numerical control system including such a switch apparatus.
2. Description of the Related Art
Generally, a machine tool uses a computerized numerical control apparatus (hereinafter, referred to as “numerical control apparatus”) for movement control of a tool or the like. In order to monitor the safety of the machine tool, it has been common to input duplicated safety input signals to the numerical control apparatus, and to monitor the safety function of the machine tool based on a result of comparison between states of the two safety input signals. FIG. 8 is a view for explaining a conventional technique for full monitoring of the machine tool based on safety input signals generated by a push-button switch. As illustrated in FIG. 8, two contacts 351 and 352 are provided in the push-button switch 300, and a first output signal and a second output signal which are composed of a “high” and a “low” signal, respectively, are inputted to the numerical control apparatus 200 as the first and second safety input signals. In the numerical control apparatus 200, the states of the first and second safety input signals are compared, and when the two safety input signals are non-coincident, it is determined to be in a state in which the safety function of the machine tool controlled by the numerical control apparatus 200 is impaired (in other words, the machine tool is abnormal).
When a safety input signal is duplicated using the push-button switch 300 as illustrated in FIG. 8, due to characteristics specific to the respective contacts 351 and 352 as well as respective signal lines, there is a difference between times from when the contacts 351 and 352 are depressed until when the numerical control apparatus 200 receives the first safety input signal and the second safety input signal (hereinafter referred to sometimes as “response times”). In other words, even when the machine tool is normal, there is a time zone in which the states of the two safety input signals received by the numerical control apparatus 200 are non-coincident, and due to such non-coincidence of the safety input signals, the numerical control apparatus 200 may erroneously determine that “the machine tool is abnormal”. Therefore, in order to avoid such erroneous determination, the numerical control apparatus 200 is set up such that a non-coincidence is permitted for a predetermined time. Hereinafter, the time for which it is permitted that the safety input signals are non-coincident is referred to simply as “permissible time”.
For example, as described in Japanese Unexamined Patent Publication (Kokai) No Hei 11-161326, there is known a numerical control apparatus that monitors receiver circuits for inputting machine information, wherein two external signals received by the independent receiver circuits are compared, and the receiver circuits are monitored based on a result of the comparison.
FIG. 9 is a view illustrating by way of example a difference in response time between the safety input signals generated by the push-button switch illustrated in FIG. 8. For example, as illustrated in FIG. 9, when the push-button switch 300 is depressed at a time 0, assuming that the numerical control apparatus 200 receives, at a time t1, the first safety input signal transmitted from the contact 351 and receives, at a time t2, the second safety input signal transmitted from the contact 352, the difference between the response times is ΔT (t2−t1). However, depending on the structure of the push-button switch 300, a case could occur in which the difference ΔT in response time between the duplicated safety input signals becomes so great that it exceeds the permissible time P. In this instance, the numerical control apparatus erroneously determines that the safety function is impaired.